The present invention relates in general to a high-energy particle imaging, and more particularly, to a method of high-energy particle imaging by computing a difference between sampled pixel voltages.
Conventionally, either photographic emulsions or electronic image sensor based cameras using down-converting scintillator screens are used in a transmission electron microscope (TEM). The scintillator (or phosphor) screen converts the impinging electron image into a visible light image that can be recorded with the photon sensitive devices. These existing detection techniques have several drawbacks, such as limited sensitivity, limited resolution, poor usability, and time inefficiency.
Photographic film has been a long-standing standard for electron imaging due to the very high modulation transfer and a large field of view that can be provided. The cumbersome post-acquisition steps associated with film have lead to near complete replacement of the technique, however, by electronic recording methods.
Charged coupled device detectors (CCDs) are now widely used in electron microscopy. These detectors overcome the time-consuming steps of loading, unloading, processing, and digitizing film by providing a digital output directly. Commonly available CCD detectors have formats up to 4096 by 4096 pixels (4K×4K), although they fall short of delivering the full resolution anticipated by the pixel count alone. CCD detectors require the use of a fluorescent scintillation screen to convert the electron image to a photon image. Unfortunately, with each primary electron event, the size of a fluorescent spot produced within the scintillation screen is much larger than the CCD pixel size. Although scintillator material layer thickness can be reduced to minimize the spot size, sensitivity is sacrificed as the number of photons produced per incident electron is also reduced. For example, in electron microscopy at 300 KeV, the full width at half maximum of the spot from a typical scintillator material is about 30 μm. However, the full width at 1% is 200 μm. The large spread of light reduces the effective resolution of the CCD camera, which often has pixels on the order of 15 μm, by at least a factor of two in each dimension, thus rendering the effective resolution of a 4K CCD camera to 2K×2K. This is far less than the resolution achieved by film, which for a 8 by 10 cm sheet is on the order of 8K×10K.8K. Nevertheless, except in rare instances, CCDs have replaced film because of other shortcomings associated with film.
A new type of detector for TEM that overcomes the limitations of scintillators and may deliver, or even exceed, the full resolution of film is disclosed in U.S. Pat. No. 7,262,411 entitled “Direct Collection Transmission Electron Microscopy” the complete contents of which are incorporated herein by reference. A detector based on active pixel sensors is used in direct bombardment mode to achieve direct detection of primary electrons without use of a scintillator screen. This new detector includes a pixel array comprising charge collection diodes that collect secondary electrons generated when a primary electron passes through the thin epitaxial silicon layer in which the p-n junction of the diode is formed. These detectors may achieve relatively high-speed readout, high spatial resolution, and very high sensitivity to single primary electrons.
A drawback of this and many other detectors currently used for electron imaging is that they intrinsically record quantities of energy deposited by the incident particles as they traverse a thin sensing volume in the detector. These thin-section detectors, which happen also to include photographic film and scintillator screens, collect a small fraction of the primary particle energy as it scatters inelastically (losing some energy as it scatters) while passing through the sensing volume.
By contrast, some detectors used in x-ray and high-energy particle detection, rely on completely stopping the particle and collecting all of its energy, by using relatively thick sensing volumes. While these thick detectors can accurately (with relatively low noise) record the energy of the incident particle, they suffer from poor spatial resolution due to the lateral scattering the incident particle can undergo in the sensing volume, and do not prove to be very good imaging detectors. Reducing the sensing volume thickness of the detector improves spatial resolution, but results in lower collected signals in the thin-section detectors. Additionally, the signal in the thin-section detectors is highly variable, due to the fact that the energy deposited by the incident particle in the thin sensing volume varies statistically according to the Landau distribution, which has a long tail extending to high energy (up to the total energy of the incident electron). This varying nature of the deposited energy in the thin-section detectors introduces an additional SNR penalty that is particularly apparent under low dose imaging conditions.
Counting methods have been proposed for use with both bulk detectors (also known as hybrid detectors) and thin-section detectors such as CCDs, active pixel sensors, and strip detectors. Counting avoids the noise associated with integrating methods by simply recording either the presence or absence of an incident electron in a pixel. To be successful however, counting requires very high frame rates if practical electron beam intensities are to be used. Counting methods also afford the opportunity to improve spatial resolution by performing analysis of the cluster of pixels that receive charge from an incident electron.
Conventional read-out methods used with active pixel sensors are considered too slow, however, for large arrays to be used with electron beams of practical intensities. This limitation arises because of the need to clear charge from the array between exposure frames.
Any pixel in an active pixel imaging-sensor is able to detect input signals (from light, electrons, etc.) by integrating ionization electrons in its sensing volume and collecting the total charges in its photodiode. During the pixel reset, the photodiode of each pixel is connected to a reset voltage and the charge in the photodiode is cleared out. At the completion of the reset, the photodiode enters an integration mode and starts to collect ionization electrons that are generated from an external source, such as light or electron illumination.
Referring now to FIG. 1 there is depicted a symbolic illustration of a photodiode voltage response profile of a single pixel to a constant external signal input (constant illumination) and a reset pulse profile (with photodiode voltage represented along the vertical axis and time represented along the horizontal axis). The external signal drives the diode voltage down linearly with increasing integration time. When the pixel is reset again, the voltage recovers (exponentially) and returns back to the reset level. It is noted that the voltage may actually be driven up (depending upon the particular pixel design setup utilized).
Referring now to FIG. 1, a prior art method to read such a sensor is to sample the voltage output of the photodiode (also referred to as photodiode voltage and pixel voltage) at the end of each integration period such as at time points as indicated at time instances 1′ and 2′. Another prior art readout method is to sample the voltage both at the beginning and end of each integration period (time instances 1, 1′, and 2, 2′). The latter readout method is often called Correlated Double Sampling (CDS), because for each pixel reset the photodiode voltage is sampled twice and only the difference between the two sampled voltages is used for the final image. The CDS method effectively eliminates the reset noise (kTc noise), thus is a preferred method for low-noise applications.
In the electron counting case, a single electron strikes a pixel and deposits a quantity of charge almost instantaneously. Referring now to FIG. 2 there is depicted a symbolic illustration of a photodiode voltage response profile resulting when an incident electron hit the pixel and a reset pulse profile (with photodiode voltage represented along the vertical axis and time represented along the horizontal axis). FIG. 2 shows the photodiode voltage profile when an incident electron hits the pixel in the middle of the integration time period after the first reset pulse. The photodiode voltage drops down to the low level after the electron hit (as indicated at a time instance in the middle of the integration time interval between time instances 1 and 1′), and it only begins to recover when the next pixel reset is asserted Oust after the time instance 1′).
Referring now to FIG. 3 there is depicted a sample plot of voltage values of a video output recorded from a single pixel using the CDS method as plotted over the course of 50 frames. A high-energy electron event is easily observed in frame 22. The CDS method requires two reads for each frame and a relatively long reset period, to ensure accurate quantification of the signal charge collected.
As illustrated by both FIG. 1 and FIG. 2, the voltage recovery process that occurs during the assertion of the pixel reset follows an exponential curve characterized by a reset time constant. The reset time constant is specific to the device design and is associated with the reset voltage, resistance, and capacitance of the circuitry. The reset time constant can become particularly problematic in large arrays and in some cases can require reset periods as long 10 to 30 microseconds. In high frame rate applications, such as electron counting for TEM, this time constant can become an important bottleneck to attaining high frame rates.
Accordingly, there exists a need in the art for an improved high-energy particle imaging used with an active pixel direct bombardment detector in comparison to the prior art.